Crossbows typically include a bow assembly mounted on a stock portion, which includes a string latch and trigger assembly for holding and release of a drawn crossbow string. Crossbows may also include one or more cams and/or pulleys, and often multiple cables which can be held below the shooting axis by a portion of the stock.
Crossbows generally are configured in various sizes ranging from 31″ to 42″ in length and 20″ to 30″ in width. The length and width dimensions for a crossbow are important to archers. Crossbows having reduced dimensions are preferable, due to the ease of handling, cocking, and aiming, where a number of crossbows have a length of 34″ long or less, and 20″ wide.
Crossbows having a reduced overall length may have a limited power stroke. Maximizing crossbow power stroke and simultaneously reducing the overall length of the crossbow may be problematic. The power stroke of many crossbows may range from 9″ to 20″ and the industry average is 13.5″. Every inch of power stroke enables an increase in the speed of the velocity of the projectile (about 25 fps/inch), and it is not uncommon for a crossbow to achieve 330 fps with about 150 lbs of maximum pull force on the crossbow string.
There are two well accepted methods for launching a bolt from a modern crossbow. One method employs a track type crossbow design. The other method employs a trackless design.
In the track type crossbow design, a bolt shaft rests in a track located in the stock of the crossbow in the full drawn cocked position. The bolt is launched from the crossbow by being pushed down the track with the bowstring and the bolt both maintaining intimate contact with the track until the bolt has cleared the crossbow. The bolts used in this type of crossbow are usually blunt at the rear end of the bolt. The bowstring that propels the bolt simply pushes against the blunt end to propel the bolt from the crossbow.
In the trackless type crossbow design, the bolt is supported on a rest towards the front of the bolt shaft and the rear of the bolt is supported by being nocked to the bowstring in the same manner as is used in conventional bows.
Some crossbows utilize one or more cams which have progressed from simple variable leveraging units consisting of circular shapes mounted eccentrically, to more complex shapes that are intended to create more energy storage for a given power stroke.
One consideration resulting from the use of cams on a crossbow is the risk of non-linear loading at the nock end of the projectile. The use of radically profiled cams may result in discrepancies in cam timing. A discrepancy in cam timing on a compound crossbow may cause the cam with the most mechanical advantage to pull the attached bowstring in the direction of the advantaged cam. The bowstring in turn, may impart a horizontal force to the end of the projectile shaft at an angle relative to the direction of the intended bolt travel.
The trackless crossbow design is more susceptible to the effects of the cams not being properly synchronized because the projectile is only supported at its front and is intimately attached to the bowstring at the rear or nock end of the bolt. In some cases, a bolt supported in this manner can become free of the front support prior to the rear end of the bolt clearing the bow during launch. Unfortunately, the rear end of the bolt is free to be acted upon by the external forces exerted by the bowstring as soon as it clears the trigger assembly. As a result, any cam synchronization problem that causes the bowstring to be pulled in one direction or the other during the launch of the bolt will have a tendency to displace the nock end of the bolt horizontally in the same direction. This results a corresponding degree of erratic projectile flight.
Given the adverse effects on projectile flight that can result from a lack of synchronization between twin cams on a crossbow, it would be desirable to have a crossbow that does not require synchronization and reacts in a consistent fashion during bolt launch without imparting unwanted forces to the rear end of the bolt.
It is desirable to provide a crossbow capable of increased mechanical efficiency and subsequent arrow launch speed while also being more pleasurable for an archer to use, requiring less maintenance, having a shorter width between the limbs as measured axle-to-axle between cams or rotation members.
In the past archers have used handheld compound bows incorporating one or more complex shaped cams to simultaneously increase arrow speed, and to provide a desired let-off, to assist an archer in the holding or retention of a bowstring in a drawn position during aiming and prior to the release of a bowstring to shoot an arrow. In the past experimentation has occurred concerning the optimal amount of let-off for a handheld compound bow at draw. The results of the experimentation has identified that a direct relationship exists between the amount of let-off for a handheld compound bow and the amount of torque which occurs on a bow as let-off is increased. In this instance, the torque at issue refers to non-linear forces applied to the bow by the archers hand as it pushes against the riser, which results in a twisting force inadvertently being applied to the bow. In a high let-off compound bow design, torque is increased. In a high let-off compound bow design an archer will frequently grasp a handle exerting an unequal or lateral pressure on the handle creating torque, which is out of alignment relative to the shooting plane for the bow. As the let-off for the handheld compound bow increases, the torque and misalignment of the bow relative to the shooting plane increases, resulting in an inaccurate arrow flight. A balance has been made between torque for a handheld compound bow, the desired shooting accuracy, as well as the let-off of the bow at draw.
As a result of the inaccuracies resulting from increased let-off and increased torque, bow manufactures have purposely limited the amount of let-off for a handheld compound bow. Typically, commercial handheld compound bows have a let-off in the range of 60% to 80%. Handheld compound bow manufacturers have known that the provision of a let-off in excess of 80%, and the associated torque and inaccuracy of an arrow flight is undesirable, which reduce the performance of the handheld compound bow to an acceptable level. Bow manufacturers have therefore determined that increasing the let-off for a handheld compound bow above 80% is undesirable, and creates an excessive and unacceptable level of torque, degrading the shooting accuracy and performance for the compound bow.
Therefore, in the past, it has been known that in compound bows, it is highly desirable, if not imperative, to restrict the level of let-off for a compound bow to regulate the undesirable effects of torque on shooting accuracy.
To the extent that a compound crossbow exhibits let-off, the amount of let-off is typically less than the let-off found in handheld bows. In a handheld bow, a higher amount of let-off will reduce the pull force required to maintain the bow at full draw—thus, a high let-off handheld bow can be easier to hold and shoot. In crossbows, the archer does not provide the force to maintain the crossbow string at full draw, so shooter fatigue does not encourage higher amounts of let-off.
All US patents and applications and all other published documents mentioned anywhere in this application are incorporated herein by reference in their entirety. Without limiting the scope of the invention a brief summary of some of the claimed embodiments of the invention is set forth below. Additional details of the summarized embodiments of the invention and/or additional embodiments of the invention may be found in the Detailed Description of the Invention below. A brief abstract of the technical disclosure in the specification is provided as well only for the purposes of complying with 37 C.F.R. 1.72. The abstract is not intended to be used for interpreting the scope of the claims.